checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/transdecoder/package.py

@@ -16,7 +16,7 @@ class Transdecoder(MakefilePackage):
     url      = "https://github.com/TransDecoder/TransDecoder/archive/TransDecoder-v5.5.0.tar.gz"
 
     version('5.5.0', sha256='c800d9226350817471e9f51267c91f7cab99dbc9b26c980527fc1019e7d90a76')
-    version('3.0.1', 'f62b86a15fcb78b1dada9f80cc25f300',
+    version('3.0.1', sha256='753a5fac5bfd04466aeabff48053c92e876cece8906b96de3b72f23f86fafae7',
             url='https://github.com/TransDecoder/TransDecoder/archive/v3.0.1.tar.gz')
 
     depends_on('perl', type=('build', 'run'))